Clamping devices for holding materials to be worked upon are well known, particularly in the woodworking industry where a workpiece is held on a jig. Clamps that use a cam-shaped surface to engage and position a clamping surface are particularly preferred as they allow for quick clamping of a workpiece with a single motion.
Prior art examples of such cammed clamping devices are shown in the following U.S. Patents:
U.S. Pat. No. 2,908,303 to Schmidt, Jr. teaches a clamping device that uses a locking handle to rotate spaced cam portions.
U.S. Pat. No. 2,402,165 to Clutter discloses a work clamp that has a pivoting clamping lever having distal cam portions that are used to tighten and release the clamp.
U.S. Pat. No. 423,992 to Froelich discloses a conventional screw clamp having a cam lever to tightly clamping the material as a final adjustment.
U.S. Pat. No. 2,639,744 to Herbert teaches an adjustable cam actuated clamp.
U.S. Pat. No. 2,043,125 to Stahl provides a cammed clamping device that can be adjusted by virtue of manually rotatable nut atop the threaded shaft on which the clamping member of the device moves.
The chief disadvantage present in existing cam action clamp designs as exemplified by the above prior art is that no easy means are provided to adjust the clamp during the process of actually clamping a workpiece. It is an inevitable result of using cammed clamping devices that adjustments must be made in the distance between the gripping surfaces of the clamp in order to accommodate workpieces of different sizes. A clamp controlled by cammed actuating means exerts a maximum clamping force at a definite point in the rotation of the cam, and the cam is only able to move the clamping surfaces together by a specified distance determined by the size of the cam lobes. Therefore, clamps that use a camming action must be adjustable on a coarse scale independent of the camming action to handle different sized workpieces whereupon the cammed clamping action can be used as a fine scale adjustment to obtain the desired clamping force.
Generally, the prior art devices rely on fairly complex coarse adjustment means that make operation of the clamp when setting up for a new size of workpiece awkward. For example, Stahl's clamping device has a coarse adjustment means comprising a knurled knob atop a threaded shaft. The design is such that when the cam action clamp is used adjustment of the knurled knob is prevented so that adjusting the clamp for a new workpiece becomes a trial and error process of adjusting the knurled knob, testing the cammed clamping action for adequate grip, then releasing the cammed clamp and making further adjustments to the coarse adjustment through the knurled knob. Admittedly, a skilled worker would probably be able to accomplish this task in one or two trials, but a less frequent user would likely require more trials resulting in wasted time and effort each time a workpiece is changed.